Presbyopia

Presbyopia is part of the natural aging process of the eye, and can be easily corrected. Technically, presbyopia is the loss of the eye's ability to change its focus to see objects that are near. It is not a disease. It's as natural as wrinkles, and it affects everybody at some point in life. Presbyopia generally starts to appear around age 45.

Presbyopia is often confused with farsightedness , but the two are different. Presbyopia occurs when the eye's lens loses flexibility. Farsightedness occurs as a result of the shape of the eyeball, which causes light rays to bend incorrectly once they have entered the eye.

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Source: MedicineNet.com
http://www.medicinenet.com/presbyopia/article.htm